In truth, there are no definitive tests for mental illness. Mental illness is an abstract concept determined by a relationship with a hypothetical normative brain. In this sense, mental illnesses are not like other diseases in that there is no objective state in which a person can be said to have a specific illness. The primary way in which mental illnesses are diagnosed is through the expertise of a professional, and not all professionals diagnose mental illnesses in the same ways. There are, however, certain basic features that can be used as tests for mental illness when interpreted intelligently.
The most basic tests for mental illness are questionnaires in which the patient answers questions about his or her condition, resulting in a report on possible symptoms. These tests for mental illness are only accurate if they are well designed and properly interpreted. Problems with questionnaires might result from misunderstanding the questions, lying, or asking questions that do not apply to the situation. As a starting point, simply asking about symptoms can be one of the best tests for mental illness. Sometimes, a person who feels that he or she is suffering from a mental illness can benefit from an online questionnaire because it can be taken from the privacy of one's own home without any professional help, but these are not definitive diagnoses.
Visual tests are sometimes used to diagnose specific mental illnesses, such as schizophrenia. People with certain disorders sometimes see images differently than people who do not suffer from mental illness. One example is the hollow mask illusion, which fails to trick some people with schizophrenia. These tests are not always definitive, particularly if a person is familiar with the test, but they are strong diagnostic tools.
Most of the time, observation of a person's demeanor is used as a strong indicator of mental illness. People who act in abnormal ways or who display unusual thought processes often have mental illnesses. Through interaction and observation, a professional can determine whether a mental illness exists by comparing symptoms to official criteria.
There are also some tests that are not commonly used but that could be used in the future for a definitive diagnosis of mental illness. Blood tests that look for genes associated with mental illness could be used to diagnose mental illnesses that do not yet present symptoms. Brain scans can sometimes be used to point out abnormal functions in the mind. Using these tests, though, requires a radical redefining of what constitutes mental illness, as the current consensus on the subject is that, without symptoms, a mental illness cannot exist.